Dynamic allocation of frequency spectrum

ABSTRACT

A method and arrangement in a communication system for dynamically allocating communication system frequency spectrum after cross-talk interface between transmission medias used for upstream and downstream communication in the frequency spectrum. After detecting cross-talk interface on a disturbed transmission medium caused by transmission on a disturbing transmission medium, the arrangement transmit a media control signal on the disturbed transmission medium and detects a reproduced media control signal that is reproduced due to cross-talk on the disturbing transmission medium. The disturbed or disturbing transmission media can then be dynamically allocated different frequency spectrum to eliminate the interface. In one embodiment, each transmission medium has a default transmission direction, and if one of the transmission is in a direction opposite the default direction, that transmission is reassigned to another transmission medium.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to methods and arrangements fordynamically allocating communication system frequency spectrum afterinterference between transmission medias using the frequency spectrum.

DESCRIPTION OF RELATED ART

Resource handling and resource utilization are fundaments of any moderncommunication. In the case of Very-high-speed Digital Subscriber LineVDSL modems, this fundamental issue is represented by frequencyplanning. The frequency planning in VDSL has to take many aspects intoconsideration: political, economical, and practical. Different frequencyplans fit different services favored by one or the other operator andpractical issues introduce many side constraints. Examples of thesepractical aspects are the location of HAM bands for radio amateurs andthe frequency plans of other broadband systems, for instanceAsymmetrical Digital Subscriber Line ADSL. Most common systems of today,including VDSL, use fixed allocation of the frequency spectrum. Thisleads to under-utilization of the medium.

Signaling in the copper plant is subject to cross-talk, which means thatcommunication on a given line is disturbed by signals transmitted onother lines. The coupling between lines in general increases withfrequency but can vary substantially from line to line. However, intoany given line, the cross-talk is usually strong only from a smallnumber of other lines.

Modern Digital Subscriber Line DSL systems are designed to be limited byeither near-end cross-talk NEXT or far-end cross-talk FEXT. For FEXTlimited systems, frequency planning is necessary in order to controlcross-talk, that is, to avoid NEXT. While NEXT-limited systems do notneed any spectrum management, they could still benefit fromtransmit-power management. In FEXT-limited systems, upstream anddownstream traffic is typically assigned to different frequency bands.Without this frequency division duplexing FDD, the NEXT would severelyreduce the available capacity in the network. As an analogy to this typeof multi-user interference, we can compare with the situation in acellular radio-communication system. The copper lines in a DSL systeminterfere significantly with only a few neighboring lines and therebyfunction as directional antennas do in a cellular system. However,cellular radio systems usually have much more sophisticated resourceallocation mechanisms than only FDD. Correspondingly, the lack ofefficient resource allocation mechanisms in the copper plant leads to asignificant under-utilization of the available resources. In a U.S. Pat.No. 6,167,095 is disclosed a method for variably allocating upstream anddownstream communication spectra. In the US patent, upstream anddownstream portions of the frequency spectrum are allocated based oncomputed signal quality parameters representative of the lineconditions. The signal must be characterized before the allocation offrequency bands in the spectra takes place. Chaotic resource allocationwill potentially lead to system breakdown. To avoid system breakdown,coordination between the modems is necessary. Without the presentinvention, this in general requires a central site determining theupstream and downstream carrier frequencies depending on the line noisecharacteristics.

SUMMARY OF THE INVENTION

The present invention solves problems related to dynamic and efficientallocation of frequency spectrum without using a central coordinatingdevice. Using cross-talk coupling between medias in order to make someof the medias observant of a possible disturbing situation solves theproblem. The invention uses the fact that interference and cross-talkcoupling is symmetric in the sense that if interference arises on adisturbed first transmission media due to cross-talk coupling oftransmission on a disturbing second transmission media, a cross-talkcoupling also exists in the converse direction. Due to the symmetry inthe cross-talk coupling, transmission on the first media will arise viathe cross-talk coupling as interference on the second media. Accordingto the invention, a media control signal is sent on the disturbed mediaafter cross-talk interference. The media control signal then reproducesand is detected on the disturbing media. Depending on whether theinvolved medias are used in a defined default direction or in adirection opposite the default direction, different measures will betaken.

More in detail in a first embodiment, cross-talk interference isdetected on a disturbed transmission media used by a disturbed set ofmodem pair in a communication system. A disturbing transmission media,used by a disturbing set of modem pair in the system, causes theinterference due to a cross-talk coupling. A media control signal issent by the disturbed set of modem pair on the disturbed media. Themedia control signal is reproduced on the disturbing media due to thecross-talk coupling and is detected by the disturbing set of modem pair.

More in detail in a second embodiment, collision avoidance data istransmitted in resources of frequency spectrum on a disturbingtransmission media by a receiving-end modem of a disturbing set of modempair. The purpose of the collision avoidance data is to avoid collisionin the form of strong NEXT interference. Cross-talk interference isdetected on a disturbed transmission media used by a disturbed set ofmodem pair. The collision avoidance data on the disturbing media causesthe interference due to a cross-talk coupling. A media control signal issent by the disturbed set of modem pair on the disturbed media. Themedia control signal is reproduced on the disturbing media due to thecross-talk coupling and is detected by the disturbing set of modem pair.

More in detail in a third embodiment, modem-pairs in the system that areinfluenced by interference are synchronized in time. Collision avoidancedata is then simultaneously transmitted by receiving-end modems of socalled disturbing set of modem pairs at predefined first set oftimeslots in the frequency spectrum on disturbing transmission medias.Cross-talk interference is detected on a disturbed transmission mediaused by a disturbed set of modem pair. The collision avoidance data fromone of the disturbing medias caused the interference. A media controlsignal is sent at a predefined second set of timeslots by the disturbedset of modem pair on the disturbed media. The media control signal isreproduced on the disturbing media and is detected by the disturbing setof modem pair.

More in detail in a fourth embodiment, a further modem-pair, a so-callednon-disturbing modem-pair detects a second reproduced media controlsignal. The second control signal is reproduced from the alreadymentioned media control signal sent by the disturbed set of modem pairon the disturbed media (see the earlier embodiments). The media controlsignal comprises in this embodiment information about transmissiondirection of the disturbed transmission media and is reproduced on thenon-disturbing media due to a second cross-talk coupling. Due to thecarried transmission direction information, the non-disturbingmodem-pair can choose to either ignore the signal or to take measures tohandle the situation.

The object of the present invention is to use capacity in the network inthe sense that users in the system dynamically can seize the capacitythey need if this disturbs no one else.

One advantage with the invention is that capacity can be allocatedflexibly and according to need.

Another advantage is that no central coordinating device is needed.

Yet another advantage is that it is possible to increase traffic loadwithout loosing control over the system. For moderate overall trafficloads, media utilization can be much better than static frequencyplanning. With increasing traffic load, the performance goes downtowards that of a static frequency planning.

The invention will now be described more in detail with the aid ofpreferred embodiments in connection with the enclosed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a block schematic illustration of a communication systemcomprising modem pairs having receiving end modems and sending endmodems.

FIG. 1B shows the frequency spectrum divided into an example set ofdifferent frequency channels each having a defined default direction.

FIG. 2 shows a block schematic illustration of a communication systemwith a disturbing modem pair and a disturbed modem pair, each modem pairhaving a receiving end modem and a sending end modem. Signalingdirections in the figure disclose the signaling in a first embodiment.

FIG. 3 shows a flow chart illustrating the first embodiment when anintention signal is used.

FIG. 4 shows a block schematic illustration of a communication systemwith two modem pairs. Signaling directions in the figure disclose thesignaling in a second embodiment when collision avoidance data has beensent.

FIG. 5 shows a block schematic illustration of a communication systemcomprising modem pairs. Signaling directions in the figure disclose thesignaling in a third embodiment of the invention whereby the modems havebeen synchronized in time.

FIG. 6 shows a block schematic illustration of a communication systemcomprising a disturbing modem pair, a disturbed modem pair and anon-disturbed modem pair, in a fourth embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1A discloses an example broadband wireline communication system.The system in FIG. 1A is a copper wire installation that comprises acentral office CO or a cabinet where modems are co-located. It is to benoted that this is an example and that the invention is not restrictedto wireline systems. Additionally, instead of having a central office,more important is that modems are within cross-talk range from eachother. The system comprises modem pairs A1-A2, B1-B2, C1-C2, D1-D2,E1-E2, F1-F2, G1-G2, H1-H2, each pair including a receiving end modemand a sending end modem. The modem pairs communicate on transmissionmedia in either an upstream or downstream direction. In FIG. 1A, it canbe seen that modem ends A1, B1, C1, D1, E1, F1, G1, H1 of the modempairs are co-located while the other modem ends A2, B2, C2, D2, E2, F2,G2, H2 are at different locations. When a sending end modem transmits,it may cause near-end cross-talk NEXT to receiving end modems locatedclose to it when using the same frequency band. Assume, for example,that the sending end modem H1 transmits to its receiving end modem H2.If the modem pair F1-F2 at the same time is using the same frequencyband in the opposite direction, a receiving end modem F1 most likelywill be disturbed by NEXT.

FIG. 1B discloses with arrows an example of how default transmission isdefined for different frequency bands. From a resource sharingpoint-of-view in a time-asynchronous system, the relevant measure ofwhat resources a modem uses for transmission is the distribution ofpower over the frequency band. When one modem wants to change the powerand/or frequency allocation used on the line, then some controlinformation describing the new transmission setup could be communicatedto the other end. This control information may be communicated on a pairof pre-allocated control channels, for instance the two lowermost bandsCH1 and CH2. Since in the present scenario there is no central resourcecontroller, it is desirable to simplify the spectrum management scheme.One suitable way of doing this is to pre-define a set of frequencybands, each having a default transmission direction, i.e. eitherupstream or downstream. One example of a possible frequency layout isshown in FIG. 1B. The two lowermost bands CH1 and CH2 in the figure areset up for fixed use (i.e., in these bands it is never allowed totransmit in the opposite direction to the default). All other bands CH3,CH4, CH5 and CH6 are set up for a dynamic upstream or downstream use. Amodem is allowed to freely use a frequency band in the defaultdirection, possibly under some power restrictions. Under some harderrestrictions, a modem can be allowed to use some frequency bands also inthe reverse of the default direction. The idea is to allow a single userto use all available resources if no one else needs them at a givenpoint in time. A modem that is using a frequency band in its oppositedirection must be prepared to back off from that band if someone elsewants to use it in the default direction.

As soon as resources are not needed any more, they should be released toallow other users to access them. Obviously this will cause variationsin the cross-talk on the line. All users will however benefit from asignificant reduction of the average interference level compared to thecase when all modems are transmitting with a constant power whether theyhave data to transmit or not.

A prerequisite for the invention is that one or several resources ineach frequency band regularly are set aside. These resources could beeither sub frequency bands or time slots. Such a (small) resource could,for instance, be a few consecutive symbols out of a thousand. That is,after each thousand symbols, a small number of symbols (such as four)could not be used for regular data transmission. For instance, after onethousand symbols have been transmitted, one symbol period could be leftsilent, the next used in the opposite direction of the one thousandfirst, one more silent, and the another thousand symbols in the firstdirection, and so forth. The small resource can also be used to listento cross-talk signals from other modems. One or several such smallresource could also be a (small) portion of the spectrum in a frequencyband. These small frequency portions of the spectrum should then alwaysor at parts of the time be used in the opposite direction. Optionally,several such small resources could be avoided when placing data. Apurpose of the unused resources and the reversing of transmissiondirection will be more obvious when the different embodiments arefurther analyzed later in the description.

FIG. 2 discloses in a first embodiment of the invention, the modem pairsA1-A2 and B1-B2 of the modem pairs already shown in FIG. 1A. Each modempair comprises a sending end modem and a receiving end modem. Two of themodem ends A1 and B1 of the modem pairs A1-A2 and B1-B2 are locatedwithin cross-talk range, as an example of location within the centraloffice CO, and the two other modem ends A2 and B2 are located outsidethe central office. All of the modem ends A1, A2, B1 and B2 compriseeach one, a transmitter TXA1, TXA2, TXB1 and TXB2 and a receiver RXA1,RXA2, RXB1 and RXB2. Each modem end A1, A2, B1 and B2 has theopportunity to become, within each frequency band, either a sending endmodem or a receiving end modem. The current traffic situation decideswhich side of the modem pair that constitutes sending end or receivingend side. The modem ends A1 and A2 are in this example connected via awire CH-X and thereby constitute the modem pair A1-A2. The modem ends B1and B2 are connected via a wire CH-Y and constitute the modem pairB1-B2. The tube form symbol T symbolizes that the wires are close toeach other within cross-talk range and that cross-talk coupling andinterference can occur. In the example that now will be discussed, themodem end A2 is transmitting on frequency channel CH4 (shown in FIG.1B). The modem end A2 thereby constitutes the sending end modem. Themodem end A1 receives the transmission from A2 and thereby constitutesthe receiving end side. As can be seen in FIG. 1B, frequency channel CH4is defined to have the upstream direction as default direction. Upstreamdirection is in this example the direction towards the central officeCO. The modem pair A1-A2 thereby communicates in the default direction.

A method according to the first embodiment of the invention will now bediscussed. The method shows how cross-talk is used to communicate with adisturbing source and to send signals from a disturbed modem pair to adisturbing modem pair. Unfilled arrow symbols in FIG. 2 representtransmission signals after having passed the cross-talk coupling. Fromnow on the modem pair A1-A2 is called the disturbed set of modem pairA1-A2 and the modem pair B1-B2 is called the disturbing set of modempair B1-B2. The method comprises the following steps:

-   -   The sending end modem A2 of the disturbed set of modem pair        A1-A2 transmits data 10 on the frequency channel CH4 to the        receiving end side A1. Due to the long distance between the end        modem B1 of the disturbing set of modem pair B1-B2 and the        sending end modem A2 of the disturbed set of modem A1-A2, B1        will not hear the transmission from A1-A2. For the same reason        the end modem B2 will not hear the transmission from A1-A2.    -   The modem B1 starts to transmit data 11 on its wire CH-Y on the        same channel CH4 as the sending end modem A2 is transmitting on.        This since B1 is unaware of the fact that the channel frequency        CH4 is already occupied.    -   The receiving end modem A1 detects cross-talk interference CTI        on wire CH-X on the disturbed channel CH4. The cross-talk        interference is caused by the transmission on wire CH-Y used by        the disturbing set of modem pair B1-B2. This due to the        cross-talk coupling T between CH-Y and CH-X.    -   The receiving end modem A1 transmits a media control signal CS        on the frequency channel CH4. As already have been mentioned        when FIG. 1 was discussed, all modem pairs have the opportunity        to reverse direction during short time intervals. A possible        variation would be that also A2 transmits a media control        signal.    -   The media control signal CS is reproduced from wire CH-X to wire        CH-Y and becomes a reproduced media control signal CS′. This due        to fact that interference is symmetric in the sense that if        there is cross-talk coupling from CH-Y to CH-X, there is also        cross-talk coupling in the opposite direction, from CH-X to        CH-Y.    -   The reproduced media control signal CS′ is detected by the        disturbing sending end modem B1.    -   The disturbing set of modem pair B1-B2 leaves the channel        frequency CH4. The disturbing modem pair is using the frequency        channel CH4 in a direction opposite the defined default        direction (see FIG. 1B) and is forced to take the consequences        of this and leave the channel. Another possible scenario would        be that the frequency channel CH4 was defined as having the        opposite direction as default direction. The set of modem pair        A1-A2 then used the frequency channel CH4 in a direction        opposite the default direction while the modem pair B1-B2 was        using the channel in the defined default direction. In this case        the modem pair A1-A2 will leave the channel CH4. Optionally a        media control signal can be sent by A1-A2 indicating that it is        most desirably for A1-A2 to keep the channel. If the pair B1-B2        then has the possibility to find another channel, this may        happen.

As an alternative to the shown embodiment, B1 instead, before startingto transmit its data 11, sends an intention signal IS in one resourcewithin CH4, thereby announcing its intention to start transmitting. Themodem pair A1-A2 can then follow the above pattern before actual datatransmission by B1-B2 commences. This may prevent potential loss of datacaused by collision. This alternative will now be exemplified by anexample below.

In FIG. 3 the most essential steps of the described first embodimentwhen using the intention signal is disclosed in a flow chart. The flowchart is to be read together with the earlier shown figures. The mostessential steps are as follows:

-   -   The frequency channels CH3-CH6 in the frequency spectrum are        defined regarding default directions. This is shown in FIG. 3 by        a block 101.    -   The sending end modem A2 transmits data 10 on the frequency        channel CH4 to the receiving end side A1. The modem pair A1-A2        is using the channel in the defined default direction. This is        shown in the figure by a block 102.    -   The modem B1 intends to start transmitting data 11 on its wire        CH-Y on the same channel CH4 as the sending end modem A2 is        transmitting on. The modem pair B1-B2 will use the channel in a        direction opposite the defined default direction. Before        starting to transmit the data, B1 sends an intention signal IS        in the direction opposite the default direction. This is shown        in the figure by a block 103.    -   The receiving end modem A1 detects cross-talk interference CTI        on wire CH-X on the disturbed channel CH4. The cross-talk        interference is caused by the intention signal on wire CH-Y.        This is shown in the figure by a block 104.    -   The receiving end modem A1 transmits a media control signal CS        on the wire CH-X. This is shown in the figure by a block 105.    -   The media control signal CS is reproduced from wire CH-X to wire        CH-Y and becomes a reproduced media control signal CS′. This is        shown in the figure by a block 106.    -   The reproduced media control signal CS′ is detected by the        disturbing sending end modem B1. This is shown in the figure by        a block 107.    -   The disturbing set of modem pair B1-B2 leaves the channel CH4.        This is shown in the figure by a block 108.

FIG. 4 discloses in a second embodiment of the invention, the modempairs A1-A2 and B1-B2 already shown in FIG. 2. A method according to theinvention will now be disclosed in connection with FIG. 4. In themethod, the end modem B2 is acting as sending end modem and transmitsdata 11 to its receiving end modem B1 on the frequency channel CH5. Themethod will show, like before, how detection of cross-talk is used tocommunicate with a disturbing source and to send signals from adisturbed set of modem pair to a disturbing set of modem pair. The modempair A1-A2 is like before called the disturbed set of modem pair and themodem pair B1-B2 is called the disturbing set of modem pair. The methodcomprises the following steps:

-   -   As mentioned, the sending end modem B2 transmits data 11 to its        receiving end modem B1 on the wire CH-Y. Neither A1 nor A2 is        affected by cross-talk due to the long distance from B2 to both        A1 and A2. If the end modem A1 has the intention to start        sending to its receiving end modem A2 on the same frequency as        B2 is using, i.e. CH5, the cross-talk affecting B1 most likely        will be catastrophically strong. This situation is to be        avoided.    -   The receiving end modem B1 now temporarily becomes sending end        modem and transmits during a short time interval collision        avoidance data CAD in a resource of the frequency spectrum on        channel CH5 in direction towards B2 which now temporarily has        become receiving end modem. As already have been mentioned        earlier in the description, also receiving end modems        occasionally during short time intervals regularly can become        sending end modems. This temporarily change will occur        regularly.    -   The end modem A1 detects cross-talk interference CTI on wire        CH-X on the channel CH5. The cross-talk interference is caused        by transmission of collision avoidance data on wire CH-Y, due to        the cross-talk coupling.    -   The end modem A1 of the disturbed set of modem pair A1-A2        transmits a media control signal CS on the frequency spectrum        CH5. A possible variation would be that also A2 transmits a        media control signal.    -   The media control signal CS is reproduced from wire CH-X to wire        CH-Y and becomes a reproduced media control signal CS′. This due        to fact that interference is symmetric in the sense that if        there is cross-talk coupling from CH-Y to CH-X, there is also        cross-talk coupling in the opposite direction from CH-X to CH-Y.    -   The reproduced media control signal CS′ is detected by the        disturbing receiving end which now again is modem B1.    -   Like earlier mentioned, different scenarios now can take place        depending on if modem pair A1-A2 or modem pair B1-B2 are        communicating in the default direction or in a direction        opposite the default direction.

FIG. 5 discloses a third embodiment of the invention. The modem pairsalready shown in FIG. 4 are also shown in FIG. 5. A third modem pairC1-C2 can also be seen in the figure. The third modem pair C1-C2 is ofthe same type as A1-A2 and B1-B2 and is thereby considered to alreadyhave been described. It is of course possible to have also other modempairs in the system but in an attempt to make the figure clearer othermodems have been left out. Transmission on media CH-X, CH-Y, CH-Z is inthis embodiment divided into timeslots. A first set of timeslots isdefined for collision avoidance data and a second set of timeslots formedia control signals. Collision avoidance data is then transmitted inthe predefined set of timeslots in the frequency spectrum of disturbingtransmission medias, by receiving-end modems of so called disturbing setof modem pairs. In this example, end modems B2 and optionally C2 are thesending end modems sending regular data, while the end modems B1 and C1are the receiving end modems sending collision avoidance data CAD. Themethod comprises the following steps:

-   -   Collision avoidance data CAD are transmitted at predefined first        set of timeslots of the frequency spectrum of the medias CH-Y,        CH-Z, in this example by the receiving-end modems B1, C1.    -   Cross-talk interference CTI is detected at the predefined first        set of timeslots on a disturbed media CH-X by the disturbed        receiving end modem A1. The interference is received from a        disturbing media CH-Y used by a disturbing set of modem pair        B1-B2. This due to the sending of collision avoidance data CAD        by B1 and due to a cross-talk coupling T.    -   A media control signals CS is transmitted at a predefined second        timeslot of the frequency spectrum by the disturbed receiving        end modem A1 on the disturbed media CH-X.    -   A reproduced media control signal CS′ is detected at the        predefined second timeslot on the disturbing media CH-Y by B1 of        the disturbing set of modem pair B1-B2.

In order to avoid destructive NEXT interference, all involved modempairs A1-A2, B1-B2 and C1-C2 in the system that are subject tointerference should be synchronized in time. A description of symbolsynchronization in a DMT system with cross-talk interference can befound in the European patent application EP 1093248. All embodimentswhere frequency bands have been divided into sub bands are applicablealso when the frequency bands have been divided into timeslots.

FIG. 6 discloses in a fourth embodiment, an example of a situation wherea non-disturbing media accidentally is affected by the media controlsignal. In FIG. 6, the modem pairs A1-A2 and B1-B2 are shown togetherwith a third modem pair C1-C2. A1 and B1 are within cross-talk rangefrom each other as well as A1 and C1, while B1 and C1 are not withincross-talk range from each other. A1-A2 is called the disturbed modempair. B1-B2 is called the disturbing modem pair, and C1-C2 is called thenon-disturbing modem pair.

The situation that now will be discussed follows the earlier shownprocedure discussed in the first embodiment and shown in FIG. 2, i.e.the situation when A2 transmits data 10 in an upward direction, and B1transmits data 11 in a downward direction. A1 detects cross-talkinterference CTI due to the cross-talk coupling T between CH-Y and CH-Xand transmits a media control signal CS. However, due to a secondcross-talk coupling T2 between CH-X and a non-disturbing media CH-Z usedby the non-disturbing modem pair C1-C2, the media control signal is alsoreproduced to CH-Z and becomes a second reproduced media control signalCS″. C1 hereby assumes that it has disturbed a modem pair—which is nottrue. In order to avoid this situation, the media control signal carriesinformation about the current transmission direction of the disturbedmodem pair A1-A2 that was sending the signal.

In the frequency sub band case, the media control signal carriesinformation about the current transmission direction of the disturbedmodem pair by configuration of sub bands in the frequency band in thespectrum. In the time synchronized case, the media control signalinstead demonstrates the direction by the way the time slot pattern isconfigured when sent by the disturbed modem pair. If the indicatedtransmission direction is the same direction as the direction the modempair C1-C2 is using, the non-disturbing modem pair C1-C2 will ignore thesecond reproduced media control signal CS″.

It is to be noted that a possible scenario would be that a reproducedmedia control signal is detected only on the non-disturbing media butnot on the disturbing media. This, if for example the disturbing mediafor some reason is disturbed or turned off before the reproduced mediacontrol signal is detected.

Different variations are of course possible within the scope of theinvention. The media control signal can for example be repeated on thedisturbed transmission media by the signal sending modem. This is forredundancy purpose and prevents unintentional loss of the signal due toa disturbance before it has been received. In another example, in orderto point out to other transmission medias in the communication systemthat a sudden change of transmission direction is about to take place inthe system, a reproduced media control signal CS′, CS″ received by atransmission media is forwarded to at least one other closely relatedtransmission media in the system. The reproduced media control signal isin this way propagated through the communication system.

The invention is in other words not limited to the above described andin the drawings shown embodiments but can be modified within the scopeof the enclosed claims.

1. A method in a communication system for dynamically allocatingcommunication system frequency spectrum after detecting cross-talkinterference (CTI) on a disturbed transmission medium, wherein the CTIis caused by transmission on a disturbing transmission medium due to across-talk-connection between the media, said method comprising thesteps of: transmitting a media control signal on the disturbedtransmission medium between a first modem and a second modem as a resultof the previously detected CTI; detecting a reproduced media controlsignal on the disturbing transmission medium by a third modem or afourth modem, said reproduced media control signal being reproduced fromthe media control signal transmitted on the disturbed transmissionmedium due to the cross-talk connection; and dynamically allocating bythe third and fourth modems, a different frequency spectrum to thedisturbing transmission medium to eliminate the CTI.
 2. The method in acommunication system for dynamically allocating communication systemfrequency spectrum according to claim 1, wherein the frequency spectrumis divided into frequency bands, each band being divided into sub bands.3. The method in a communication system for dynamically allocatingcommunication system frequency spectrum according to claim 1, whereinthe frequency spectrum is divided into frequency bands, each band beingdivided into timeslots.
 4. The method in a communication system fordynamically allocating communication system frequency spectrum accordingto claim 1, wherein the CTI comprises an intention signal transmitted onthe disturbing transmission medium before transmission of regular data.5. The method in a communication system for dynamically allocatingcommunication system frequency spectrum according to claim 1, whereinthe CTI comprises collision avoidance data regularly transmitted on thedisturbing transmission medium in a direction opposite the transmissiondirection of regular data.
 6. The method in a communication system fordynamically allocating communication system frequency spectrum accordingto claim 1, wherein the frequency spectrum is divided into frequencybands and each band has a defined default direction.
 7. The method in acommunication system for dynamically allocating communication systemfrequency spectrum according to claim 1, wherein the media controlsignal is repeated on the disturbed transmission medium, for redundancypurpose.
 8. The method in a communication system for dynamicallyallocating communication system frequency spectrum according to claim 1,wherein the first modem transmits the media control signal to the secondmodem, and the second modem repeats the media control signal back to thefirst modem on the disturbed transmission medium.
 9. The method in acommunication system for dynamically allocating communication systemfrequency spectrum according to claim 1, further comprising the step of:detecting a second reproduced media control signal on a non-disturbingtransmission medium, reproduced due to a second cross-talk connection.10. The method in a communication system for dynamically allocatingcommunication system frequency spectrum according to claim 1, whereinthe media control signal comprises information about transmissiondirection of the disturbed transmission medium.
 11. The method in acommunication system for dynamically allocating communication systemfrequency spectrum according to claim 1, further comprising the step of:repeating the reproduced media control signal received by a transmissionmedium, with the purpose of forwarding it to another, at least one,transmission medium in the system.
 12. A method in a communicationsystem for dynamically allocating communication system frequencyspectrum after cross-talk interference between modem pairs, each pairbeing used for upstream and downstream communication via transmissionmedias in the frequency spectrum, said method comprising the steps of:detecting by a disturbed modem pair, cross-talk interference (CTI) on adisturbed medium used by the disturbed modem pair, said interferencebeing caused by transmission on a disturbing medium used by a disturbingmodem pair, due to a cross-talk coupling; transmitting a media controlsignal by the disturbed modem pair on the disturbed medium, as a resultof the previously detected CTI: detecting by the disturbing modem pair,a reproduced media control signal on the disturbing medium, reproduceddue to the cross-talk coupling; and dynamically allocating by thedisturbed modem pair, communication system frequency spectrum to thedisturbed transmission medium to eliminate the CTI.
 13. A method in acommunication system for dynamically allocating communication systemfrequency spectrum after cross-talk interference between modem pairs,each pair being used for upstream and downstream communication viatransmission media in the frequency spectrum, said method comprising thesteps of: regularly transmitting collision avoidance data on a firstchannel of the frequency spectrum on a disturbing medium by areceiving-end modem of a disturbing modem pair; detecting cross-talkinterference (CTI) on a disturbed medium used by a disturbed modem pairoperating on the first channel, said interference being caused bycross-talk coupling due to transmission of the collision avoidance dataon the disturbing medium by the receiving-end modem; transmitting amedia control signal by one of the modems of the disturbed modem pair onthe disturbed medium, as a result of the previously detected CTI;detecting by the disturbing modem pair, a reproduced media controlsignal on the disturbing medium, reproduced due to the cross-talkcoupling; and changing to a second channel of the frequency spectrum byeither the disturbing modem pair or the disturbed modem pair, whereinthe first channel has a default direction of transmission, and the modempair that changes to the second channel is the pair that is transmittingin a direction opposite to the default direction of transmission.
 14. Amethod in a communication system for dynamically allocatingcommunication system frequency spectrum after cross-talk interferencebetween modem pairs in the system, each pair being used for upstream anddownstream communication via a plurality of transmission media in thefrequency spectrum, said method comprising the steps of:time-synchronizing all modem pairs in the system that are subject tocross-talk interference; periodically transmitting by synchronizedreceiving-end modems, collision avoidance data at a predefined first setof time slots in the frequency spectrum on each of the transmissionmedia; detecting cross-talk interference at a predefined first set oftimeslots on a disturbed medium used by a disturbed modem pair, saidinterference being caused by transmission on a disturbing medium used bya disturbing modem pair, due to a cross-talk coupling; transmitting amedia control signal at a predefined second set of time slots by thedisturbed modem pair on the disturbed medium, as a result of thepreviously detected cross-talk interference; and detecting by thedisturbing modem pair, a reproduced media control signal, reproduced dueto the cross-talk coupling, at the predefined second set of time slotson the disturbing medium.
 15. A method in a communication system fordynamically allocating communication system frequency spectrum aftercross-talk interference between transmission media used for upstream anddownstream communication in the frequency spectrum, said methodcomprising the steps of: detecting by a disturbed modem, cross-talkinterference on a disturbed transmission medium, said interference beingcaused by transmission by disturbing modems on a disturbing transmissionmedium due to a cross-talk-connection between the media; transmitting bythe disturbed modem, a media control signal on the disturbedtransmission medium as a result of the previously detected cross-talkinterference; and detecting by at least one other modem, a reproducedmedia control signal on at least one other transmission medium, saidreproduced media control signal being reproduced from the media controlsignal due to at least one cross-talk coupling between media.
 16. Themethod in a communication system for dynamically allocatingcommunication system frequency spectrum after cross-talk interferenceaccording to claim 15, further comprising the step of: detecting asecond reproduced media control signal on a non-disturbing transmissionchannel, reproduced due to a second cross-talk coupling.
 17. Anarrangement in a communication system for dynamically allocatingcommunication system frequency spectrum after cross-talk interferencebetween transmission media used for upstream and downstreamcommunication in the frequency spectrum, said arrangement comprising:means for detecting cross-talk interference on a disturbed transmissionmedium said interference being caused by transmission on a disturbingtransmission medium due to a cross-talk-connection between the media;means for transmitting a media control signal on the disturbedtransmission medium as a result of the previously detected cross-talkinterference; means for detecting a reproduced media control signal onthe disturbing transmission medium, reproduced from the media controlsignal due to the cross-talk connection; and means for dynamicallyallocating different frequency spectrum to the disturbed or disturbingtransmission media to eliminate the CTI.
 18. The arrangement in acommunication system for dynamically allocating communication systemfrequency spectrum after cross-talk interference according to claim 17,further comprising means for transmitting an intention signal on thedisturbing transmission medium before transmission of regular data. 19.The arrangement in a communication system for dynamically allocatingcommunication system frequency spectrum after cross-talk interferenceaccording to claim 17, further comprising means for transmittingcollision avoidance data on the disturbing transmission medium in adirection opposite the transmission direction of regular data.
 20. Thearrangement in a communication system for dynamically allocatingcommunication system frequency spectrum after cross-talk interferenceaccording to claim 17, further comprising means for repeating the mediacontrol signal on the disturbed transmission medium, for redundancypurpose.
 21. The arrangement in a communication system for dynamicallyallocating communication system frequency spectrum after cross-talkinterference according to claim 17, wherein the means for transmittingthe media control signal includes means for sequentially transmittingthe media control signal from each end of the disturbed transmissionmedium to an opposite end.
 22. The arrangement in a communication systemfor dynamically allocating communication system frequency spectrum aftercross-talk interference according to claim 17, further comprising meansfor detecting a second reproduced media control signal on anon-disturbing transmission medium.